Conventionally, many electric/electronic devices mounted to a vehicle such as an automobile are grounded to a negative terminal of a battery via the vehicle body by fixing the negative terminals of the devices to a metallic vehicle body (body ground). Such a ground terminal is disclosed in JP H6-60061A (Patent Document 1), for example. Also, in cases where the vehicle body is made of a non-conductive material such as a carbon-fiber reinforced plastic in order to meet such demands as making the vehicle lighter, the negative terminals of the devices and the negative terminal of the battery are sometimes grounded by a wire.
Incidentally, research regarding automatic vehicle driving has grown in recent years. It is important that power failure does not occur in the vehicle, and it is necessary that the grounded state of the ground terminal does not deteriorate over time. Conventionally, ground terminals are bolted at a constant pressure, using a torque wrench.
However, there is an issue with just using a torque wrench to fasten a bolt at a constant pressure in that it cannot be denied that the grounded state of a ground terminal may deteriorate as the bolt loosens over time. Thus, for example, in WO 2014/208654 (Patent Document 2), a terminal provided with a temperature sensor for monitoring the connection state of a terminal by sensing heat generated by the terminal has been proposed. However, even if this terminal is applied to the ground terminal, it is difficult to reliably detect loosening of the terminal because most of the heat escapes via the vehicle body, and it cannot be said that this is a sufficiently desirable measure to take.